Background stability and annual modulation test using PICOLON ultra-pure NaI(Tl) detector

TL;DR

This work reports the first six-month annual modulation search using PICOLON’s ultra-pure NaI(Tl) detectors to test the DAMA/LIBRA signal. It demonstrates six-month radiopurity stability of Ingot#94 produced by the Ingot#85 purification route, and provides a detailed background model including internal RI concentrations and external sources, complemented by advanced noise-reduction strategies. Using a DM-rate framework with Earth’s motion and quenching factors, the analysis places a spin-independent WIMP-nucleon cross-section limit near $1\times10^{-41}$ cm$^2$ for $m_\chi \sim 100$ GeV/$c^2$ in the 2–6 keVee ROI, approaching the DAMA region under certain assumptions. The study shows PICOLON’s viability for verifying the DAMA result and outlines planned improvements (double-readout detector and machine-learning noise reduction) to further suppress backgrounds and refine sensitivity.

Abstract

The dark matter observation claimed by the DAMA/LIBRA experiment has been a long-standing puzzle within the particle physics community. NaI(Tl) crystals with radiopurity comparable to DAMA/LIBRA's are essential for adequate verification. Existing experiments using NaI(Tl) target have been hampered by the high radioactivity concentration of NaI(Tl) crystals. PICOLON experiment conducts an independent search for Weakly Interacting Massive Particles using highest purity NaI(Tl) crystals. In 2020, the NaI(Tl) crystal (Ingot#85) reached the same purity level as DAMA/LIBRA crystals. The DAMA/LIBRA group has stressed that verifying their signal requires high-purity NaI(Tl) crystals with long-term stability. Based on a six-month measurement, we have confirmed the long-term stability of its radiopurity. This stability provides a significant advantage for future efforts to adequately verify the DAMA/LIBRA result using NaI(Tl) crystal. In this paper, we present the background stability of purity in the Ingot#94 NaI(Tl) detector, which was produced using the Ingot#85 purification method, along with the first annual modulation search conducted by the PICOLON experiment.

Figures (13)

• Figure 1: Schematic view of the PICOLON shield (length unit: mm).
• Figure 2: Ingot#94 $R_\mathrm{PSD}$ result for the electron equivalent energy. Red dash line is $\mathrm{\alpha}$-ray events extraction threshold.
• Figure 3: $\alpha$-ray energy spectrum. The horizontal-axis represents the calibrated energy of alpha-ray emissions. Six peaks are observed, corresponding to RIs in the Th- and U-series. The number of events is estimated assuming the radioactive equilibrium.
• Figure 4: An excess peak is observed due to RI contamination from the external NaI(Tl) crystal. The PMTs and the oxygen-free copper shielding are considered the primary sources of the contamination.
• Figure 5: Example of typical waveform at 4 $\mathrm{keV_{ee}}$ signals. a vertical axis is a normalized amplitude of waveforms. (a) NaI(Tl) scintillation event signal. (b) single pulse noise event signal. (c) baseline noise event signal has $\Delta t>1000~ns$, where $\Delta t$ is the time difference between two single pulses from the start time of the primary pulse.